In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of radio frequency (RF) current through the gas. In the original class of HID lamps, discharge current was caused to flow between two electrodes. However, a major cause of early electroded HID lamp failure has been found attributable to at least two inherent operational characteristics of such lamps. First, during lamp operation, sputtering of electrode material onto the lamp envelope is common and reduces optical output. Second, thermal and electrical stresses often result in electrode failure.
Electrodeless HID lamps do not exhibit these life-shortening phenomena found in electroded HID lamps. One class of electrodeless HID lamps involves generating an arc discharge by establishing a solenoidal electric field in the gas; and, hence, these lamps are referred to as HID-SEF lamps. In an HID-SEF lamp, the discharge plasma or fill is excited by RF current in an excitation coil surrounding the arc tube. The HID-SEF arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. In particular, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a changing magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, thus producing a toroidal arc discharge in the arc tube.
For efficient lamp operation, the excitation coil must not only have satisfactory coupling to the discharge plasma, but must also have low resistance and small size. A practical coil configuration permits only minimal light blockage by the coil and hence maximizes light output. A conventional excitation coil is of a long solenoidal shape. However, another excitation coil configuration is disclosed in U.S. Pat. No. 4,812,702 issued on Mar. 14, 1989 to J. M. Anderson and assigned to the instant assignee. The excitation coil of the cited patent, which is hereby incorporated by reference, has at least one turn of a conductor arranged generally upon the surface of a toroid with a rhomboid or V-shaped cross-section that is substantially symmetrical about a plane passing through the maxima of the toroid. Still another type of excitation coil for an HID-SEF lamp is described in commonly assigned copending U.S. patent application Ser. No. 240,331 of H. L. Witting, filed on Sept. 6, 1988 now U.S. Pat. No. 4,894,591, which incorporated by reference. The Witting application describes an inverted excitation coil comprising first and second solenoidally-wound coil portions, each being disposed upon the surface of an imaginary cone having its vertex situated within the arc tube or within the volume of the other coil portion.
Despite the advantages offered by HID-SEF lamps, luminaires for housing HID-SEF lamps which allow for both efficient operation and easy lamp replacement have been heretofore unknown. Accordingly, it is an object of the present invention to provide such a luminaire.
Another object of the present invention is to provide an HID-SEF luminaire which has an excitation coil attached thereto and allows for easy lamp replacement, the new luminaire being simple in construction and easy to fabricate.
Still another object of the present invention is to provide an HID-SEF luminaire which houses an easily replaceable HID-SEF lamp with light reflecting means for maximizing light output from the lamp arc tube.
Yet another object of this invention is to provide an HID-SEF lamp, including starting electrodes, which is easily and conveniently replaceable in a luminaire.